On 28 June 2018 at 10:50, Peter Zijlstra [off-list ref] wrote:

On Wed, Jun 27, 2018 at 06:06:01PM +0200, Ard Biesheuvel wrote:

quoted

diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h
index 86ec0652d3b1..aa203dffe72c 100644
--- a/include/linux/jump_label.h
+++ b/include/linux/jump_label.h

@@ -121,6 +121,32 @@ struct static_key {
 #include <asm/jump_label.h>

 #ifndef __ASSEMBLY__
+#ifdef CONFIG_HAVE_ARCH_JUMP_LABEL_RELATIVE
+
+struct jump_entry {
+     int code;
+     int target;
+     int key;
+};

I much prefer you use 'u32' there.

Actually, they are signed so that would be s32. But yeah, I can change that.

quoted

+static void jump_label_swap(void *a, void *b, int size)
+{
+     long delta = (unsigned long)a - (unsigned long)b;
+     struct jump_entry *jea = a;
+     struct jump_entry *jeb = b;
+     struct jump_entry tmp = *jea;
+
+     jea->code       = jeb->code - delta;
+     jea->target     = jeb->target - delta;
+     jea->key        = jeb->key - delta;
+
+     jeb->code       = tmp.code + delta;
+     jeb->target     = tmp.target + delta;
+     jeb->key        = tmp.key + delta;
+}
+
 static void
 jump_label_sort_entries(struct jump_entry *start, struct jump_entry *stop)
 {

@@ -56,7 +72,9 @@ jump_label_sort_entries(struct jump_entry *start, struct jump_entry *stop)

      size = (((unsigned long)stop - (unsigned long)start)
                                      / sizeof(struct jump_entry));
-     sort(start, size, sizeof(struct jump_entry), jump_label_cmp, NULL);
+     sort(start, size, sizeof(struct jump_entry), jump_label_cmp,
+          IS_ENABLED(CONFIG_HAVE_ARCH_JUMP_LABEL_RELATIVE) ? jump_label_swap
+                                                           : NULL);
 }

That will result in jump_label_swap being an unused symbol for some
compile options.

No, and isn't that the point of IS_ENABLED()? The compiler sees a
reference to jump_label_swap(), so it won't complain about it being
unused.

Would it not be much nicer to write that like:

static void jump_label_swap(void *a, void *b, int size)
{
        struct jump_entry *jea = a, *jeb = b;

#ifdef CONFIG_HAVE_ARCH_JUMP_LABEL_RELATIVE
        long delta = a - b;

        jea->code += delta;
        jea->target += delta;
        jea->key += delta;

        jeb->code -= delta;
        jeb->target -= delta;
        jeb->key -= delta;
#else

        swap(*jea, *jeb);
}

And then unconditionally use jump_label_swap().

Meh. I thought IS_ENABLED() was preferred over #ifdef, no? That way,
the compiler always sees the code, and simply discards it without
complaining if it ends up left unused.